Optical information recording and reproducing apparatus

ABSTRACT

An optical information recording and reproducing apparatus comprises an optical disk having a reflection layer, an optical information recording layer, a super resolution layer and a protection layer on the recording surface of a substrate, a laser emission control unit for emitting the laser light to record and reproduce the optical information and changing the pulse-like emission pattern, a pickup for radiating by focusing the laser light and receiving the reflected light, a spindle and a spindle motor for supporting and rotationally driving the optical disk, and a reproduction signal processing unit for arithmetically processing the received signal.

BACKGROUND OF THE INVENTION

This invention relates to an optical information recording andreproducing apparatus, or in particular to an optical informationrecording and reproducing apparatus high in recording density and havinga high super resolution effect.

With the recent progress of the information society using opticaltelecommunication, the construction of a telecommunication systemcapable of high-speed communication of information high in recordingdensity has been required. An optical information recording andreproducing apparatus capable of accumulating the optical information ofhigh recording density is an indispensable optical device for developingthe high-speed optical telecommunication of high recording density.Further, with the digitization of video information such as TV imagesand the increased image quality such as high definition, it is of urgentnecessity to develop an information recording and reproducing apparatusof high recording density capable of long-time recording whilemaintaining a high image quality.

Currently, a DVD having a capacity of 4.7 GB on each side finds wideapplications as an optical information recording medium for handlinghigh-density dynamic images such as computer and video information. Thepractical application of this DVD as a rewritable recording andreproducing medium as well as a read-only ROM (DVD-ROM) with informationwritten directly in the substrate has been promoted. The development ofthese optical information recording media is aimed at a high recordingdensity, and the laser light having a shorter wavelength of 650 nm thanthe laser (780 nm) for the CD is used as a means to achieve the highdensity information recording. For applications to the computer graphicsand the digital high-definition images involving the information oflarge recording capacity, however, the recording density four to fivetimes higher is required. In order to meet this requirement, an opticaldisk using a blue semiconductor laser still shorter in wavelength (405nm) and having the recording density of 23.3 GB on one side has beendeveloped and found practical applications.

As a technique to further increase the recording density of the opticaldisk, the development of a multilayer recording method, a multi-valuedrecording method and a super resolution recording method is under way.Of these next-generation methods to achieve a high recording density,the super resolution recording method is one of the most promisingtechniques.

In the super resolution recording method, the waist of the laser beamradiated on the recording surface is reduced using the laser focusingfunction or the masking function of the super resolution layer. This isone of the recording methods of high density recording realized by thereversible change in the optical constants (refractive index (n) and theextinction coefficient (k)) of the super resolution layer formed in amultilayer structure such as the recording layer, the protection layeror the reflection layer of the optical disk. The super resolution layer,upon radiation of the read/write laser thereon, is excited by thetemperature rise or the absorption of photons of light. As long as thelaser is radiated, therefore, the refractive index and the extinctioncoefficient are changed reversibly, while the original state is restoredupon extinction of the laser. In the optical disk, a recording portionand a non-recording portion are determined for reproduction by theamount of the laser light returned to the pickup after being radiatedand reflected on the optical disk. Due to this reversible change of theoptical constants in the super resolution layer, the area of the lightreturned to the pickup can be reduced more than the normal radiationarea of the laser light. Specifically, the resolution can be improved byreducing the readable area using the optical masking effect.

The extinction coefficient (k) is a quantity proportional to the lightabsorption coefficient of a material, and assumes a larger value thelarger the absorption coefficient of the material. The two constantsincluding the refractive index (n) and the absorption coefficient (k)are collectively called the optical constants.

In the prior art, as described in JP-A-10-340482, for example, a thinfilm material of cobalt oxide has been used as the super resolutionlayer. The large change in refractive index and the super resolutioneffect of this thin film can produce an optical disk of high recordingdensity.

In the optical information recording and reproducing apparatus currentlyavailable, the optical information is reproduced by radiating thecontinuous wave (CW) light or the reproducing laser light superposedwith high frequencies of about 400 MHz. In the case where the thecomplex refractive index of the super resolution layer is changed andthe power of the reproducing light is increased until the superresolution effect is obtained, therefore, heat is accumulated on theoptical information recording medium by the radiation of laser light,and a broad heat distribution occurs in the laser beam spot, therebyposing the problem that a super resolution mask high in contrast cannotbe formed. Further, the heat accumulation degradate the film or therecording pit on the medium, resulting in the degradation of therepetitive reproducing operation characteristic.

A reproducing method with pulse light for improving the super resolutioneffect by avoiding the heat accumulation in the medium and steepeningthe temperature gradient in the beam spot is described, for example, inJP-A-10-40547.

In an application of the pulse reproducing method as described inJP-A-10-40547, a high response of the material is essential. Also, thebeam spot in super resolution state, which is a superposition of a beamspot in ground state and a beam spot in super resolution state, isaffected by the beam spot in ground state, and a satisfactory superresolution mask is difficult to obtain.

SUMMARY OF THE INVENTION

The object of this invention is to provide an optical informationrecording medium formed with a super resolution film having a highresponse and a thermal stability and an optical information recordingand reproducing apparatus capable of producing a high super resolutioneffect using the medium.

In order to achieve the object described above, according to thisinvention, there is provided an optical information recording andreproducing apparatus comprising at least a substrate, a reflectionlayer formed on the recording surface of the substrate, an opticalinformation recording layer, a super resolution layer having the complexrefractive index reversibly changing with temperature, an opticalinformation recording medium having a protection layer, a laser emissioncontrol unit for emitting the laser light adapted to record or reproducethe optical information in the optical information recording medium andto change the laser emission pattern, a pickup for radiating by focusingthe laser light on the surface of the optical information recordingmedium and receiving the light reflected from the optical informationrecording medium, a spindle and a spindle motor for supporting androtationally driving the optical information recording medium, and areproduction signal processing unit for arithmetically processing thereceived signal,

wherein the laser light is emitted in pulses by the laser emissioncontrol unit, and the reproduction signal processing unit acquires thereproduction signal from the pulse emitting portion and the biasemitting portion of the pulse light and produces, as a reproductionsignal, the result of arithmetically processing the reproduction signalfrom the pulse emitting portion with reference to the reproductionsignal in the bias emitting portion.

The arithmetic process described above is intended to produce thedifference between the reproduction signal from the pulse emittingportion and and a constant multiple of the reproduction signal from thebias emitting portion. Also, the arithmetic process is executed afterdetermining the reproduction signal of the bias emitting portion at thesame time point as the reproduction signal of a given pulse emittingportion by interpolating the reproduction signals of the adjoining biasemitting portions temporally before and after the reproduction signal ofthe particular given pulse emitting portion.

The pulse emitting portion of the super resolution layer is in superresolution state but not the bias emitting portion thereof.

The super resolution state is defined as a state in which a part of thelaser spot is masked and the optical resolution is increased by thechange in the optical constants due to the laser radiation and theresulting temperature increase of the super resolution layer. The statenot in the super resolution state, on the other hand, is called theground state.

The super resolution layer contains Fe₂O₃, Co₃O₄, NiO, CoO, ZnO, Cr₂O₃,ZnS—ZnSe, GaN—InN and Ga₂O₃, and preferably contains Fe₂O₃ and Ga₂O₃.Further, the optical information recording layer is a metal filmconfigured of one or more metal elements selected from Ag, In, Ge, Sband Te.

The optical information recording and reproducing apparatus using thepulse reproducing method according to this invention can reproduce, witha high efficiency, the optical information recording medium formed witha super resolution film, and an optical disk having a high resolutionagainst a small recording mark can be fabricated. Thus, theapplicability of this optical information recording and reproducingapparatus is very high.

In the optical information recording and reproducing apparatus accordingto this invention, the laser light shaped into pulses of the emissionfrequency higher than the shortest mark frequency is radiated on theoptical information recording medium having a super resolution layer ofhigh-speed response capable of following the pulse width. At the sametime, the optical signals are detected from the pulse emitting portionand the bias emitting portion not emitting the pulse light, and theresult of the arithmetic operation performed with these signals is usedas a reproduction signal. Therefore, a super resolution mask having alarge contrast can be formed. As a result, the optical informationrecording and reproducing apparatus higher in recording density than inthe prior art can be obtained. Further, the temperature can be increasedwithout deteriorating the super resolution layer or other opticalinformation recording media, thereby producing an optical informationrecording and reproducing apparatus higher in reliability than in theprior art.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an optical information recording andreproducing apparatus fabricated by the method according to thisinvention.

FIG. 2 is a sectional view schematically showing a ROM-type opticalinformation recording medium fabricated by the method according to thisinvention.

FIG. 3 is a schematic diagram showing the light emission pattern forpulse reproduction.

FIG. 4 is a diagram showing the reproduction pattern with the pulselight and CW light.

FIG. 5 is a diagram showing the temperature dependency of thereflectivity change rate ΔR in the case where a multilayer film havingthe same structure as the ROM-type optical disk fabricated by the methodaccording to the invention is heated.

FIG. 6 is a diagram showing the reproduction power dependency of thereflection light intensity (V_(out)) on the mirror surface of theROM-type optical disk fabricated by the method according to theinvention.

FIG. 7 is a diagram showing the data of the reproduction output of thenormal reproducing unit and the pulse reproducing unit.

FIG. 8 is a diagram showing the change of the resolution of 2T, 3T and4T marks with Pr′.

FIG. 9 is a diagram in which the logarithm of the resolution against thelength (T) of each mark is plotted.

FIG. 10 is a schematic diagram showing the frequency dependency of thereproduction signal intensity in super resolution state.

FIG. 11 is a schematic diagram showing a temperature profile on the diskfilm surface and an optical mask formed in the case where the opticaldisk having the super resolution effect is reproduced by the CW lightand the pulse light.

FIG. 12 is a sectional view schematically showing the recording-typeoptical information recording medium fabricated by the method accordingto the invention.

FIG. 13 is a diagram showing an example of the recording waveform forrecording in the recording-type optical information recording mediumfabricated by the method according to the invention.

FIG. 14 is a diagram showing an example of the waveform obtained bypulse reproduction.

FIGS. 15A, 15B, 15C are diagrams showing an example of the waveform atthe time of pulse emission and the bias emission and the waveformcorrected and arithmetically processed.

FIG. 16 is a schematic diagram showing the reproduction signal in theneighborhood of the reproduction signal at the time of the pulseemission at time t+Nδ.

FIGS. 17A, 17B are diagrams showing an example of the waveform obtainedfrom the reproduction signal of bias emission and the reproductionsignal of pulse emission at time t+Nδ.

DETAILED DESCRIPTION OF THE INVENTION

An optical information recording apparatus having the recording densitymore than 1.5 times that of the prior art has been obtained by using anoptical information recording medium containing Fe₂O₃ or Ga₂O₃ as asuper resolution layer and, as a reproduction signal, the differencebetween the reproduction signal from the pulse emitting portion and thereproduction signal from the bias emitting portion, which has the pulsewidth of 3 ns, the pulse period of 8 ns, the emitting power of 6 mW forthe pulse emitting portion and the emitting power of less than 0.8 mWfor the bias emitting portion with no pulse radiation.

Embodiment 1

An optical information recording and reproducing apparatus for recordingand reproducing the optical information recording medium according tothe invention has been fabricated. FIG. 1 is a block diagram showing theoptical information recording and reproducing apparatus thus fabricated.In FIG. 1, reference numeral 1 designates an optical informationrecording medium (hereinafter referred to as the optical disk), numeral2 a spindle, numeral 3 a spindle motor, numeral 4 a motor circuitcontrol means, numeral 5 a pickup, numeral 6 a medium identifying means,numeral 7 a laser driver, numeral 8 a reproducing power DC amplifier,numeral 9 a reproducing peak power determining means, numeral 10 areproducing bias power determining means, numeral 11 a recording powerDC amplifier, numeral 12 a recording peak power determining means,numeral 13 a recording peak power ratio determining means, numeral 14 anerasing power DC amplifier, numeral 15 a reproduction signal detectionmeans, numeral 16 a peak sample means, numeral 17 a bias sample means,numeral 18 a difference reproduction signal calculation means, numeral19 an address reading means, numeral 20 a clock sync means, numeral 21 areproduction signal demodulation means, numeral 22 a reproduction datasupply means, numeral 23 a tracking error detection means, numeral 24 aninformation controller, numeral 25 a pickup control circuit, numeral 26a recording timing correcting means, numeral 27 a recording datamodulation means, numeral 28 a recording data receiving means, andnumeral 29 a pickup motion driver.

The optical information recording and reproducing apparatus according tothe invention includes the medium identifying means 6 for identifyingthe type of the optical disk 1 as a recording medium which is classifiedinto the rewritable (RW) type, the write-once (WO) type and theread-only memory (ROM). In the laser emission control system describedbelow, only the reproducing system is driven in the case where theoptical disk inserted is the ROM, the reproducing system and therecording system in the case where the optical disk inserted is of WOtype, and the reproducing system, the recording system and the erasingsystem in the case where the optical disk inserted is of RW type. Theoptical disk 1 is fixed provisionally on a rotary mechanism connected,directly or indirectly, to the rotary shaft of the spindle motor 3 fixedon the spindle 2 and controlled by the motor circuit control means 4.The information on the optical disk is read as an optical signal by thesensor for detecting the laser providing the light source of the pickup5 and the reflected light. Also, the information is stored in theoptical disk by the light source in the pickup. Also, the pickup is setin position along a track by the pickup motion driver 29.

The laser emission control unit is classified into the reproducingsystem, the recording system and the erasing system. The reproducingsystem includes the reproducing power DC amplifier 8, the reproducingpeak power determining means 9 and the reproducing bias powerdetermining means 10 for pulse reproduction according to the invention.Thus, the pulse reproduction waveform of the reproduction light isformed, and the emission pattern is sent to the laser driver 7 and thepickup 5 thereby to emit the laser in pulse form.

At the time of recording the data in the recording system, the recordingdata is input from the recording data receiving means 28 and modulatedby the recording data modulation means 27. The data thus modulated isinput to the laser driver 7 through the recording timing correctingmeans 26 thereby to control the light source in the pickup 5. The outputof the recording peak power ratio determining means 13 is input to thepickup 5 through the recording power determining means 12, the recordingpower DC amplifier 11 and the laser driver 7 thereby to control thelight source in the pickup 5. Further, in the erasing system, therecording data is input to the laser driver 7 through the erasing powerDC amplifier 14 thereby to control the light source in the pickup 5.

The optical signal obtained at the time of reproduction is processed inthe reproduction signal processing system. The optical informationdetected by the reproduction signal detection means 15 are sampled bythe peak sample means 16 and the bias sample means 17 for the pulseemitting portion and the bias emitting portion separately. These twosignals are arithmetically processed by the difference reproductionsignal calculation means 18. The signals thus processed are outputexternally by the reproduction data supply means 22 through the addressreading means 19, the clock sync means 20 and the reproduction signaldemodulation means 21. The reproduction data is output by apredetermined output means such as a display unit or a speaker orprocessed by an information processing system such as a personalcomputer.

The focal point and the focal depth vary from one optical disk toanother, and therefore, an optical disk having the auto focusingfunction was selected. Further, in keeping with the configuration inwhich the a focusing function layer is mounted on the disk and thetracking width is narrowed, a tracking error detection means 23 for highdensity recording was added to make possible the tracking suitable foran arbitrary medium. The information from the tracking error detectionmeans 23 is transmitted to the pickup 5 through the informationcontroller 24 and the pickup control circuit 25. Also, the mechanism foridentifying a medium type utilizing the reflectivity difference betweenmedia was employed to make the auto tracking possible in accordance withthe difference in medium type.

The laser light source mounted on the pickup has the wavelength of 405nm. Also, the objective lens for focusing the laser beam on the opticaldisk has the NA (numerical aperture) of 0.85.

The characteristic of the optical information recording medium havingthe super resolution effect according to the invention was evaluatedusing the optical information recording and reproducing apparatus shownin FIG. 1. First, the read-only ROM disk was evaluated. A sectional viewof the ROM-type optical information recording medium fabricated is shownin FIG. 2. In FIG. 2, numeral 31 designates a substrate, numeral 32 areflection layer, numerals 33, 35 protection layers, numeral 34 a superresolution layer, numeral 36 a cover layer, and numeral 37 a recordingpit. In the ROM-type optical information recording medium according tothis embodiment, the recording pit 37 has the function as an opticalinformation recording layer. According to this embodiment, a mediumstructure suitable for the optical system having the laser wavelength of405 nm and the numerical aperture of 0.85 was realized by using apolycarbonate substrate 31 having the outer diameter of 120 mmφ, theinner diameter of 15 mmφ and 1.1 mm thick, and the cover layer 36 of apolycarbonate sheet having the outer diameter of 119.5 mmφ, the innerdiameter of 23 mmφ and 0.1 mm thick. The reproducing laser is focusedfrom the cover layer 36 side for reproduction.

The ROM disk was fabricated by the processes described below. First, arecording pit pattern having mark spaces at predetermined intervals wasformed on a photo resist using a laser drawing unit. After that, the pitpattern was copied to a Ni die, and a substrate was formed by ejectionmolding of polycarbonate using this die. The minimum pit size was 139nm, and the pit depth 22 nm. Also, the track pitch was 320 nm.

A reflection film of an alloy containing 95% Ag, 2.5% Pd and 2.5% Cu(mol %) was formed as the reflection layer 32 on the substrate thusfabricated. The film thickness was 20 nm. The film was formed by DCmagnetron sputtering using the pure Ar gas. An amorphous film containing80% ZnS and 20% SiO₂ (mol %) was used as the protection layers 33, 35,which were formed by RF sputtering using the pure Ar gas. A filmcontaining 50% Fe₂O₃ and 50% Ga₂O₃ (mol %) was used as the superresolution layer 34. This layer was formed by RF sputtering using a gascontaining 95% Ar and 5% O₂ (flow rate %). An oxide target having thesame composition as described above was used as a sputtering target toform the super resolution layer.

After forming these layers by sputtering, the cover layer 36 was formed.The UV-curable resin was applied, by spin coating, on the substrate 1.1mm formed with a film, and a polycarbonate cover layer cut to a circlehaving the outer diameter of 119.5 mmφ, the inner diameter of 23 mmφ andthe thickness of 0.085 mm was attached thereon. The resulting assemblywas introduced into a vacuum chamber and while being deaired to a vacuumof about 1 Pa, the sheet and the substrate were closely attached to eachother. The UV-curable resin was cured by radiating the UV light from thecover layer side. The thickness of the UV-curable resin was adjusted tothe total thickness of 0.1 mm including the UV-curable resin and thecover layer.

According to this embodiment, a polycarbonate substrate 1.1 mm thick wasused as the substrate 31. The outer diameter of the substrate 1 is 120mm, and a hole having the inner diameter of 15 mm was formed for thechuck. On this substrate, the recording pit 37 for the CN ratio test wasformed of steps on the same track at predetermined period with thespace.

The recording pits corresponding to the recording signals of 2T, 3T, . .. , 8T and the spaces are repeatedly recorded with the clock signal(1T=69.5 nm). Only one type of recording signal is formed on the sametrack, and different signals are recorded on different tracks. In thiscase study, the length of the recording pit of the 2T signal providingthe shortest mark is 139 nm, and the length of the recording pit for the8T signal providing the longest mark is 556 nm.

Further, a mirror surface portion in the shape of a ring concentric withthe optical disk and having no recording pit is formed on the substrate1. This mirror surface portion is called the mirror surface. Bymeasuring the amount of light reflected from the mirror surface, thenonlinearity of the super resolution layer 34 can be evaluated.

The output of the repetitive signal described above is observed byoscilloscope, and it can be concluded that the larger the ratio(resolution) of the amplitude of the fine mark such as 2T and 3T to thesignal amplitude based on the longest mark (8T), the higher theresolution of the shortest mark. According to this invention, theresolution (Mod) was defined by Equation (1) below.

$\begin{matrix}{{Mod} = \frac{I_{npp}}{I_{8{pp}}}} & (1)\end{matrix}$where I_(8pp) is the amplitude ratio based on the longest mark andI_(npp) the amplitude ratio of the mark nT (n: 2 to 7) to be measured.

According to this invention, the reproducing operation was performed byradiating the pulse light on the optical information recording mediumshown in FIG. 2. The emission pattern of the pulse light used in thisinvention is shown in FIG. 3. In FIG. 3, character Pp designates theemitting power of the pulse emitting portion and character Pb theemitting power of the bias emitting portion. Character tp designates theemission time of the pulse emitting portion, and character tb theemission time of the bias emitting portion. According to thisembodiment, Pp is set to not less than 8 mW but not more than 6 mW andPb to not less than 0.3 mW but less than 0.8 mW. By doing so, the pulseemitting portion can maintain the super resolution layer in superresolution state, and the bias emitting portion can be maintained inground state.

Also, tp is set to 1 to 5 ns, and tb to 5 to 13 ns. Further, the linearspeed of rotation of the optical disk is set to 4.56 m/s. As a result,the time during which the shortest 2T mark is passed is 30.5 ns, and inthe case where tp is 3 ns and tb 5 ns, the sampling is possible withabout 3.8 pulses.

Generally, even with the laser spot in super resolution state, thereflectivity associated with other than the super resolution state isnot zero, and therefore, the signal not in super resolution state isadded to the signal in super resolution state. As shown by Equation (2),the output in ground state with the bias emitting portion low in laserpower and not associated with the super resolution state isarithmetically processed from the output in super resolution state withthe pulse emitting portion high in emitting power and associated withthe super resolution state. In this way, the signal output associatedwith only the super resolution state can be obtained.

The effective reproduction power Pr′ for the pulse reproducing methodwas determined using Eequation (2) below.

$\begin{matrix}{P_{r}^{\prime} = {P_{b} + {\frac{t_{p}}{\left( {t_{p} + t_{b}} \right)} \cdot P_{p}}}} & (2)\end{matrix}$

A specific example of this arithmetic operation is explained in detail.The beam spot in super resolution state is considered to be formed asthe sum of the linear beam spot not in super resolution state and thenonlinear beam spot in super resolution state. Basically, therefore,only the reproduction signal in super resolution state is obtained bysubtracting the portion of the normal beam spot (normally, Gaussiandistribution) from the whole beam spot in super resolution state. Inorder to obtain the suepr resolution state, the read laser light high inpower is radiated, and therefore the beam spot intensity of the linearportion also increases. As a more accurate arithmetic operation toestimate the beam spot in super resolution state correctly, therefore,the difference is taken between the beam spot of the linear portionmultiplied by a constant and the beam spot in super resolution state.

In the case where this reproducing method with the pulse signal is used,the highly accurate reproduction with super resolution is made possibleby always sampling the normal beam spot not in super resolution stateand referring to the nearest beam spot in super resolution state. Aspecific method is described below.

FIG. 14 shows an example of the waveform obtained by pulse reproduction.An example of reproducing the 2T repetitive signal is taken here. Assumethat an arbitrary time point is 0, an arbitrary pulse emission timepoint t₁, the bias emission time point t₂, and the pulse emission periodand the sampling period δ. The pulse emission time points are given ast₁, t₁+δ, t₁+2δ, . . . t₁+Nδ, t₁+(N+1)δ . . . . Also, the bias emissiontime points are given as t₁, t₁+δ, t₁+2δ, . . . t₁+Nδ, t₁+(N+1)δ . . . .By sampling the output at each time point, the reproduction signals atthe time of pulse emission and the bias emission are obtained. Thewaveforms obtained from only the time of pulse emission and only thetime of bias emission are shown in FIG. 15A. The waveform obtained fromthe reproduction signal at the time of pulse emission and the waveformobtained from the reproduction signal at the time of bias emission areshifted from each other by the time {t₂+Nδ)}−{t₁+Nδ}=t₂−t₁. Thewaveforms with this time shift corrected after forming each reproductionsignal are shown in FIG. 15B. The waveform obtained by the arithmeticprocess of “(reproduction signal at the time of pulse emission)−a(reproduction signal at the time of bias emission)” from the twowaveforms is shown in FIG. 15C, where a is a constant. This constant isdetermined in such a manner as to maximize the output of FIG. 15C andvaries with the disk type, signal type or the relation of previous andfollowing recording marks and spaces.

In forming the waveforms of FIG. 15B from those of FIG. 15A by thearithmetic operation shown in FIGS. 15A, 15B, 15C, the reproductionsignal at the time of bias emission may be inaccurate as it is notobtained from the output at the time point of t₁+Nδ. In such a case, thearithmetic operation shown in FIGS. 16 and 17 is performed. FIG. 16 is aschematic diagram showing the read out signal at the time of pulseemission at t₁+Nδ. The reproduction signal at the time of pulse emissionat t₁+Nδ is obtained by interpolation from the reproduction signals atthe time of bias emission before and after the pulse emission at t₁+Nδ,i.e. from the reproduction signals at the time of bias emission at timepoints t₂+Nδ and t₂+(N+1)δ. For example, the reproduction signal at thetime of bias emission at time point t₁+Nδ is determined by using theoutput at the time of bias emission at time point t₂+Nδ and the averagevalue or root mean value at the time of bias emission at time pointt₂+(N+1)δ.

FIG. 17A shows the waveform of the reproduction signal at the time ofbias emission determined in this manner and the waveform obtained fromthe reproduction signal at the time of pulse emission. Since the time iscorrected as described above, the time shift shown in FIG. 15 iseliminated. Also, the waveform obtained by the arithmetic process“(reproduction signal at the time of pulse emission)−a (reproductionsignal at the time of bias emission)” from the two waveforms obtained isshown in FIG. 17B, where a is a constant. This constant is determined insuch a manner as to maximize the output of FIG. 17B and varies with thedisk type, signal type or the relation between previous and followingrecording marks and spaces.

As described above, the aforementioned arithmetic process, which isintended to obtain the difference between the reproduction signal fromthe pulse emitting portion and the reproduction signal from the biasemitting portion, is executed preferably after determining, byinterpolation, the reproduction signal of the bias emitting portion atthe same time point as the reproduction signal of an arbitrary pulseemitting portion from the reproduction signals of the bias emittingportions immediately before and after the reproduction signal of thearbitrary pulse emitting portion. The super resolution layer is in superresolution state for the pulse reproduction emitting portion and not insuper resolution state for the bias emitting portion.

First, as a preliminary study, a multilayer film similar to the filmstructure shown in FIG. 2 was formed on a glass substrate, and thereflectivity change with the temperature increase was determined. FIG. 5shows the temperature dependency of the specific reflectivity change ΔRof the multilayer film having the film structure shown in FIG. 2. Thespecific reflectivity change ΔR was calculated from Equation (3) belowassuming that the reflectivity at each temperature is R and thereflectivity at 30° C. is Ro.

$\begin{matrix}{{\Delta\; R} = \frac{R - R_{0}}{R_{0}}} & (3)\end{matrix}$

In the film structure having the optical disk configuration shown inFIG. 2, the reflectivity was reduced with the temperature increase byheating at 350° C., and ΔR of −45.5% resulted.

FIG. 6 shows the reproduction power (Pr) dependency of the reflectionlight intensity on the mirror surface of the optical disk having thefilm structure of FIG. 2. The reflection light intensity was indicatedby the output (V_(out) (mV)) from the photodiode making up a photodetector. Also, the rotational linear speed of the disk was set to 3 to9 m/s for measurement.

In FIG. 6, the dashed straight line is based on the assumption that theV_(out) for each Pr is proportional to V_(out) for Pr of 0.3 mW. In allrotational speeds, it is understood that V_(out) increases substantiallylinearly up to Pr of about 0.7 mW, while V_(out) is reduced below thelinear line for 0.8 mW or more. In the film structure of this opticaldisk, therefore, the super resolution state is realized for Pr of 0.8 mWor more, while the ground state prevails for Pr of lower than 0.8 mW.

The smaller the rotational speed, the larger the degree of V_(out)reduction. Especially, for the rotational linear velocity of 3 m/s,V_(out) for Pr of 2.0 mW was about 50% of the value assumed for linearV_(out). With the increase in Pr or with the decrease in rotationallinear velocity, the laser light radiation amount per unit timeincreases and therefore, the temperature on the surface of the opticaldisk is considered to increase. As shown in FIG. 5, in the optical diskhaving the film structure of FIG. 2, the reflectivity decreases with thetemperature increase, and therefore, the reflectivity is considered todecrease with the temperature increase due to laser radiation.

Next, the ROM-type optical disk was actually fabricated and the superresolution effect due to the pulse radiation was studied.

FIG. 4 schematically shows the reproduction signals for the reproductionof nT marks of a single period with the pulse and the reproduction withthe normal CW (continuous wave) light. In the reproduction of nT markswith the pulse light, a plurality of pulses are radiated at each portionwhen the marks and spaces are passed. In the reproduction with thenormal CW light, on the other hand, the output changes continuously withthe reflectivity of the marks and spaces.

In the pulse reproduction method according to this embodiment, as shownin FIG. 4, the low and high levels of the pulse emitting portion aredesignated as V_(low (pulse)) and V_(high (pulse)), respectively, andthe low and high levels of the bias emitting portion with no pulseradiation as V_(low (bias)) and V_(high (bias)), respectively. Thus, theamplitude I_(npp) is defined by Equation (4).I _(npp)=(V _(high(pulse)) −V _(high(bias)))−a(V _(low(pulse)) −V_(low(bias)))   (4)

FIG. 7 shows the RF signal waveform of the 2T marks measured using thepulse reproduction method. The abscissa represents the time and theordinate the output (V_(out)). The pulse light is radiated from timepoint 0, and for the time before 0, the reproduction waveform of the CWlight is shown. The peak power P_(w) of the pulse light was set to 8 mW,and the base power P_(b) and the reproduction power Pr with the CW lightwere set to 0.8 mW. Under this condition, the reproduction amplitudefrom the 2T marks was not substantially observed from the reproductionwaveform of CW light, while the signal amplitude of the 2T marks wasclearly observed in the pulse reproduction unit. This is considered toindicate that the pulse light radiation strongly excites the superresolution film of Fe₂O₃—Ga₂O₃ into super resolution state for animproved resolution. The pulse light emission time is about 3 ns, andtherefore, the response speed of the Fe₂O₃—Ga₂O₃ super resolution filmis not more than 3 ns, thereby indicating that the change in refractiveindex follows the rise of the pulse light.

A similar evaluation was conducted also for the 3T and 4T marks. Thedependency of the resolution obtained from the pulse reproductionwaveform of 2T, 3T and 4T marks on the effective reproduction power Pr′is shown in FIG. 8. In this embodiment, the constant a shown in Equation(4) was determined for each of 2T, 3T and 4T marks. Then, the resolutionwas maximized at 1.52 for 2T, 1.36 for 3T and 1.27 for 4T. In the studybelow, an example of calculation using these values as the constants inEquation (4) is described.

With 2T mark, the resolution for Pr′ of 2.0 mW is about 5.8 times higherthan for Pr′ of 1.4 mW, and the resolution was remarkably improved ascompared with the reproduction waveform for the CW light. Similarly, asfor the 3T mark, the resolution was improved 2.5 times. The resolutionof the 2T marks for Pr′ of 2.0 mW was substantially the same as that ofthe 3T marks for Pr′ of 1.2 mW, indicating that the resolution along thelinear density was improved about 1.5 times.

The resolution of the 4T marks, on the other hand, was reduced with theincrease in Pr′. To analyze this phenomenon, the resolution for eachmark length (T) was plotted, as the result thereof is shown in FIG. 9.In FIG. 9, the ordinate represents the logarithm (dB) of the resolutionand the abscissa the mark length. In the case where Pr′ is low, theresolution decreases monotonically with the decrease in mark length.With the increase in Pr′, however, the modulation degree increases inthe area of the mark length of not more than 243 nm. Especially, it wasfound that the resolution conspicuously increases in the neighborhood of139 nm corresponding to the 2T marks. In some part of the mark lengthrange of 243 nm to 380 nm, on the other hand, the resolution was seen todecrease with the increase in Pr.

The beam spot obtained by the super resolution effect is given as thesum of the normal laser spot not in super resolution state and the laserspot in super resolution state. The frequency dependency of thereproduction signal intensity obtained by these beams is shown in theschematic diagram of FIG. 10. The reproduction output of the laser spotin super resolution state, though smaller than that of the normal laserbeam, has a smaller diameter for an improved resolution and has a highoutput up to the high frequency side. Thus, the cutoff frequencyincreases from fo to fo′. In the front aperture method, the mask isformed by reduction in reflectivity, and therefore the contribution ofthe laser spot in super resolution state is negative. In the superresolution state, therefore, the output on low frequency side is lowerthan in the normal reproduction.

On the high frequency side, on the other hand, the output from thenormal laser beam is 0 on the frequency side higher than the cutofffrequency fo. The output due to the super resolution spot, however,becomes conspicuous and improved. The boundary between the outputdecrease on the low frequency side and the output improvement on thehigh frequency side is considered fc.

The result of the experiment shown in FIG. 9 indicates that the marklength corresponding to fc is 243 nm or substantially equal to one halfof the laser beam spot diameter. In the case where the mark size is notless than one half of the laser spot diameter, the signal can beseparated even with the normal spot diameter having no super resolutioneffect, and therefore the amount of the reflected light is conspicuouslyreduced by the decrease in the laser spot size due to the superresolution phenomenon, thereby probably resulting in a reducedresolution. In the case where the mark size is smaller than one half ofthe diameter of the normal spot, on the other hand, more than two markscan be contained in the spot, and therefore, the resolution would bereduced. In view of the fact that the super resolution effect reducesthe laser spot size and the number of marks in the spots is reduced toless than two, thereby probably resulting in an improved resolution.

Next, the reason why the reproduction method using the pulse light ishigher in super resolution effect than the CW reproduction is explained.FIG. 11 is a schematic diagram showing a temperature profile on the diskfilm surface and an optical mask formed in the reproduction of theoptical disk having the super resolution effect with the CW light andthe pulse light. In the schematic diagram showing the temperatureprofile in the upper part of FIG. 11, the abscissa represents theposition on the circumference of the disk, and the ordinate thetemperature on the film surface. The laser spot is moved to positivefrom negative side in the drawing. When viewed in an arbitrary timesection as shown in the drawing, the light intensity distribution of thelaser spot constitutes a Gaussian distribution with the center atposition ro. Also, the schematic diagram of the optical mask at theposition corresponding to the temperature profile is shown in the lowerpart of FIG. 11. This diagram shows a case in which the higher thetemperature, the lower the reflectivity. The method shown in thisdiagram, in which the rear part of the laser beam is masked while thefront window is opened in the shape of a mask, is called the “frontaperture detection method”.

In the case where the CW light is radiated, the fact that the laser spotposition is moved from negative to positive side increases thetemperature of the film surface due to the continuous laser lightradiation and decreases the temperature with the decrease in the laserlight intensity. The resulting shape has a tail on negative side. Inaccordance with this temperature distribution, the optical constants ofthe super resolution film change and the reflectivity decreases. Thus,the negative area of the laser spot is masked thereby to reduce theeffective diameter of the laser spot.

The pulse laser light, on the other hand, is not radiated continuouslyas long as the pulse light emission period is sufficiently long ascompared with the pulse moving time, and therefore, the tail on negativeside is very small as compared with the CW light, thereby producing atemperature profile approximate to the Gaussian distribution. Also, thelaser light is radiated for so short a length of time that the specimenis thermally damaged only slightly, and therefore, the peak intensity ofthe laser light can be increased. As a result, the local temperature atthe pulse peak position can be increased. Generally, the higher thetemperature, the larger the amount of change in the optical constants ofthe super resolution material, and therefore, as compared with the CWreproduction, the reflectivity change is considered large. Thus, thereflectivity of the mask portion can be reduced and a super resolutionlaser spot larger in contrast is considered possible to form.

The foregoing description concerns an embodiment implemented using thearithmetic method shown in FIGS. 15A, 15B, 15C. The arithmetic processusing the method shown in FIGS. 16, 17A, 17B in similar fashion couldproduce as high a super resolution effect as in FIGS. 15A, 15B, 15C.

Embodiment 2

Next, the recording-type disk was similarly studied. A sectional view ofthe recording-type optical information recording medium thus fabricatedis shown in FIG. 12. In FIG. 12, numeral 41 designates a substrate,numeral 42 a reflection layer, numerals 43, 45, 48 protection layers,numeral 44 a super resolution layer, numeral 46 a cover layer, numeral47 a recording layer, numeral 49 a land, and numeral 50 a groove. In therecording-type optical information recording medium according to thisembodiment, the recording layer 47 has the function as an opticalinformation recording layer.

According to this embodiment, a medium structure suitable for theoptical system having the laser wavelength of 405 nm and the numericalaperture of 0.85 was realized by using the polycarbonate substrate 41having the outer diameter of 120 mmφ, the inner diameter of 15 mmφ andthe thickness of 1.1 mm and the cover layer 46 formed of a polycarbonatesheet having the outer diameter of 119.5 mmφ, the inner diameter of 23mmφ and the thickness of 0.1 mm. The reproducing laser is focused fromthe cover layer 46 side for reproduction.

The recording-type disk was fabricated through the process describedbelow. First, a reflection layer 42 of an alloy having the contents of95% Ag, 2.5% Pd and 2.5% Cu (mol %) was formed on the polycarbonatesubstrate having spiral lands and grooves on the recording surfacethereof. This layer was formed to the thickness of 200 nm by DCmagnetron sputtering using the pure Ar gas. The protection films 43, 45were formed of amorphus having the contents including 80% ZnS and 20%SiO₂ (mol %). These films were formed by RF sputtering using the pure Argas. Also, the recording film 47 was formed as a phase change recordingfilm of GeSbTe. This film was formed by RF sputtering using the pure Argas.

The recording layer 47 constituting the optical information recordinglayer can be a phase change recording film of AgInSbTe or a metal filmof at least selected one of the metal elements including Ag, In, Ge, Sband Tb as well as a phase change recording film of GeSbTe used in thisembodiment. Then, a rewritable recording medium reversibly changeablebetween amorphus and crystal can be fabricated. Also, in the case wherea multilayer recording film with a stack of Si and an alloy containingCu is used, the two elements are mixed by the radiation of the recordinglaser. and therefore, a stable WORM (write-once-read-many) recordingmedium can be fabricated.

After sputtering the aforementioned layers, the cover layer 46 wasformed. The UV-curable resin was formed by spin coating on the substrate1.1 mm thick formed with a film, and a polycarbonate cover layer 0.085mm thick cut into a circle having the outer diameter of 119.5 mmφ andthe inner diameter of 23 mmφ was attached thereon. The resultingassembly was introduced into a vacuum chamber and while being dearing upto about 1 Pa, the sheet and the substrate were closely attached to eachother. The UV-curable resin was cured by radiating the UV light from thecover layer side. The thickness of the UV-curable resin was adjusted tothe total thickness of 0.1 mm including the UV-curable resin and thecover layer.

According to this embodiment, the polycarbonate substrate 1.1 mm thickwas used as the substrate 41. This substrate 1 has the outer diameter of120 mm and is formed with an inner hole having the inner diameter of 15mm for a chuck. A guide groove having lands 49 and grooves 50 is formedspirally on this substrate. According to this embodiment, data wasrecorded only in the grooves 50. The track pitch was set to 320 nm andthe groove depth to about 22 nm.

The signal to be recorded is a repetition of recording pits and spacescorresponding to the recording signals of 2T, 3T, . . . , 8T for theclock signal (1T=69.5 nm). Only one type of recording signal is formedon the same track, and different signals are recorded on differenttracks. In this case study, the recording pit length of the signal 2Tproviding the shortest mark is 139 nm, while the recording pit length ofthe signal 8T providing the longest mark is 556 nm.

FIG. 13 shows an example of the recording waveform used for recording.In order to record a mark, a plurality of pulses are radiated forrecording by what is called the multi-pulse method. According to thisembodiment, in order to record nT marks, the output of the recordingpower P_(w) (mW) for τ_(w) seconds and the low output of P_(r) (mW) forτ_(r) seconds were repeatedly radiated in (n−1) pairs to form onerecording mark. FIG. 13 shows the case to form 4T marks. With 4T asτ_(m), the light of power P_(e) (mW) was radiated for the same timelength τ_(s) as τ_(m) thereby to record a space. This recordingoperation was performed for one round of the track having the sameradius. According to this embodiment, P_(w) was set to 7.2 mW, and P_(r)to 0.1 mW. Also, P_(e) was set to 4.0 mW.

According to this embodiment, various materials shown in Table 1 wereevaluated as the super resolution layer 44.

TABLE 1 Resolution Super resolution Resolution ratio (pulse filmcomposition CW Pulse reproduction/ No. (mol %) reproduction reproductionCW reproduction) Embodiment 1 100Fe₂O₃ 0.02 0.12 6.00 2 100NiO 0.03 0.134.33 3 100CoO 0.02 0.10 5.00 4 100Co₃O₄ 0.03 0.12 4.00 5 100ZnO 0.030.13 4.33 6 100Cr₂O₃ 0.02 0.11 5.50 7 78GaN—22InN 0.03 0.12 4.00 850Fe₂O₃—50Ga₂O₃ 0.02 0.14 7.00 9 49ZnS—51ZnSe 0.03 0.13 4.33 Comparative1 SiO₂ 0.03 0.02 0.67 example

Table 1 shows the composition of the super resolution film materialsstudied in this embodiment and the the result of studying the effect ofimproving, by the pulse reproduction method, the resolution of the 2Tmark as compared with the 8T mark. Table 1 shows the resolution at thetime of reproduction with the CW light having the reproduction power of0.5 mW, the resolution at the time of pulse reproduction with theemission power of 6 mW of the pulse emitting portion lower than therecording power and the emission power of 0.5 mW of the bias emittingportion, and the ratio of the resolution for pulse reproduction to theresolution for CW reproduction. A comparative example in which a SiO₂film exhibiting no nonlinearity is formed in place of the superresolution layer 44 is also shown. The resolution is defined as shown byEquation (1).

The first to ninth embodiments show the cases in which Fe₂O₃, NiO, CoO,CO₃O₄, ZnO, 78% GaN and 22% InN, 50% Fe₂O₃ and 50% Ga₂O₃, Cr₂O₃, 49% ZnSand 51% ZnSe and Ga₂O₃ are formed, respectively, as the super resolutionlayer 44. In all the cases, the resolution for pulse reproduction isvery high as compared with that for the CW reproduction. Also, theresolution ratio of the pulse reproduction to the CW reproduction was aslarge as 4.0 to 7.0. Especially, the resolution ratio for the thin filmof 50% Fe₂O₃ and 50% Ga₂O₃ formed as a super resolution layer was a verylarge 7.0. In the case where SiO₂ is formed as a comparative example, onthe other hand, the resolution substantially remains the same for CW andpulse reproduction, although the resolution for pulse reproduction wasslightly lower in the case under consideration.

As described above, an optical disk having a high resolution can beobtained by reproducing, using the pulse reproduction method, theoptical information recording medium formed with the super resolutionfilm material according to the invention.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An optical information recording and reproducing apparatus forrecording and reproducing information onto and from an opticalinformation recording medium which includes at least a substrate, areflection layer formed on the recording surface of the substrate, anoptical information recording layer, a super resolution layer having acomplex refractive index reversibly changing with a temperature change,and a protection layer, the apparatus comprising a laser emissioncontrol unit for emitting the laser light to record or reproduce theoptical information in the optical information recording medium andcapable of changing the emission pattern of the laser light, a pickupfor radiating by focusing the laser light on the surface of the opticalinformation recording medium and receiving the light reflected from theoptical information recording medium, a spindle and a spindle motor forsupporting and rotationally driving the optical information recordingmedium, and a reproduction signal processing unit for arithmeticallyprocessing the received signal, wherein the laser light is emitted inpulses by the laser emission control unit, and the reproduction signalprocessing unit produces a reproduction signal from the result of thearithmetic process using the reproduction signals from the pulseemitting portion and the bias emitting portion of the pulse light. 2.The optical information recording and reproducing apparatus according toclaim 1, wherein the arithmetic process produces the difference betweenthe reproduction signal from the pulse emitting portion and a constantmultiple of the reproduction signal from the bias emitting portion. 3.The optical information recording and reproducing apparatus according toclaim 1, wherein the arithmetic process is executed after determiningthe reproduction signal of the bias emitting portion for the same timepoint as the reproduction signal of an arbitrary pulse emitting portionby interpolating the reproduction signals of the bias emitting portionsimmediately before and after the reproduction signal of the particulararbitrary pulse emitting portion.
 4. The optical information recordingand reproducing apparatus according to claim 1, wherein the superresolution layer is in super resolution state for the pulse emittingportion but not in super resolution state for the bias emitting portion.5. The optical information recording and reproducing apparatus accordingto claim 1 or 4, wherein the super resolution layer contains Fe₂O₃,Co₃O₄, NiO, CoO, ZnO, Cr₂O₃, ZnS—ZnSe, GaN—InN and Ga₂O₃.
 6. The opticalinformation recording and reproducing apparatus according to claim 1,wherein the super resolution layer contains Fe₂O₃ and Ga₂O₃.
 7. Theoptical information recording and reproducing apparatus according toclaim 1, wherein the optical information recording layer is a metal filmconfigured of at least one metal element selected from Ag, In, Ge, Sband Te.